86 Proceedings. 



weight of the atmosphere as it reaches our coasts that we 

 have to look for the cause of wind and storm. 



Before we go further, it is necessary to impress upon you a 

 law which is of the utmost importance, and lies almost at the 

 foundation of scientific weather- study ; it is this: — "Stand 

 with your back to the wind, and the barometer is higher on 

 your right hand than on your left." This law was first 

 formulated by a Dutchman named Buys Ballot, and so 

 bears his name ; it is so important that I recommend com- 

 mitting it to memory, and bearing it constantly in mind for 

 all weather observations.* 



We have learned that the atmosphere over the Atlantic is 

 travelling towards us in a series of ridges and hollows of 

 pressure ; but these hollows are not nearly so often found in 

 the form of the trough that we see between waves of water, 

 as they are of a circular form — deepest, i.e., having the lowest 

 pressure at the centre, and gradually sloping up towards the 

 outer edges. 



These "depressions," as they are called, together with the 

 phenomena which surround them, have been given the name 

 of "cyclones" or "cyclonic disturbances"; not necessarily 

 because they are storms, but merely from the Greek word, 

 nvuXa^, a circle ; and when you hear a weather-man speak of a 

 cyclone, or you see the word in the weather-news of the day, 

 you may be assured that only one of these circular, low- 

 pressure systems is meant, and that there is little fear of a 

 great storm which you may have previously understood the 

 word to mean. Now, round every one of these depressions 

 the wind circulates in accordance with Buys Ballot's law. 

 Stand so that the centre, where the barometer is lowest, is 

 on your left, then the wind will be on your back, no matter 

 whether you are N., S., E., or W. of the centre ; and there is 

 always a tendency to draw a little in towards the centre, and 

 not to keep strictly parallel with the "isobars." We will 

 come to the anti- cyclones presently. 



The isobar is a line of equal barometric j)ressure, and is 



* This law is reversed in the Southern Hemisphere, and does not api^ly 

 to the equatorial regions of the Northern Hemisphere. 



